I have a problem with one of my start batteries going dead when the bus sits for for a week or more. I have two group 31 batteries and a vanner equalizer. I cut the disconnect switch off when parked yet the upper battery (24 volt) keeps going dead. The lower battery (12 volt) has a complete charge. I tested the vanner and everything seems normal. I thought if there was a 12 volt load it would kill the lower battery yet my issue is just the opposite. Any thoughts on what I am missing???

my center taps are off the top battery. Anyway, fully charge the batteries and test each cell. You might find one is bad/shorted. If not, you can use an ammeter to see if there is a constant load that you do not know about, or you could charge the battery and leave it disconnected and see if it drains down.

When I said upper and lower I was refering to the voltage, it is the bottom battery that keeps going dead. The 12 volt draw is off of the top battery and it is fully charged.They were both new batteries, I have had the same issue with the other batteries I had in there. I thought it was the way the PO had the taps off the batteries but looking at the vanner diagram it is correct. I may disconnect the vanner and see if that has any effect.

On my bus, the main disconnect does not disconnect the 12 volt stuff. It only disconnects the 24 volt stuff. Mine is wired that way intentionally to keep constant power to certain items. My Vanner Equalizer still keep the batteries balanced.

If your Vanner was added later it may not have a disconnect on the 12 volt side. If the Vanner is working properly the battery that has the 12 volt tap should not be going dead. I do believe the Vanner has to have 24 volt to work so if the 24 volt is getting cut off that could cause issues.

Easy way to check if a battery is bad-charge up both batteries to float; turn everything off in the bus; then disconnect the negative on all batteries. If a battery is bad, it will self discharge. If you have an electrical leak, then the batteries will all be about the same, and you'll have to start hunting for the electrical leak. Good Luck, TomC

I have a similar situationMy top battery dies over time and needs to have a charger slapped on it every week or twoI noticed though after long runs (few hundred miles) with headlights on it would get really low and be slow starting the next dayI haven't tracked this issue down yet...its my next project (just fixed an injector issue - hoping to be done with that)I have a couple questions - - whats a vanner (is it a stock item?)

I have 2x 8D batteries (24v) the top has a lead off the positive to make a 12v with separate disconnects

Bottom battery has always read good voltage even after long winter storageTop battery wouldnt hold a charge - so I replaced it and now have the above scenario (the battery that needs constant recharging is a new one)

I used to have that problem on trucks I ran re-man Delco alternators on. Rebuilder used aftermarket parts to rebuild and the batteries would dis-charge thru the regulator. (internal regulated) It would take about a week to run the batteries down to where it wouldn't start. Never had the problem with a Leece-Neville or a new Delco on the same truck. It had a 12v system, charging and start.

I'm new to coaches so is your charging system 12v and uses a series parallel switch for 24v start?Kevin

My issue is with the 24 volt side. I did add a disconnect to the 12 volt tap, but was still pulling about half amp discharge on the 24 volt side but nothing on the 12 tap. I will check in a week or so and see what happens.

I am starting to believe that adding a disconnects on each battery is going to be the simple answer(KISS). Finding a small draw on a 40 year old electrical system is next to impossible.

What kind of alternator are you running? My experience, with Delco 34 Si I think (big green one lol) was never good. If you have a Delco, pull the positive cable off it and see if your battery still goes dead.

While I don't understand your top battery explanation, I have one battery going dead before the other particularly when I drove at night. I sped up the problem when I installed brighter headlights. I was told that by using a center tap one battery will always discharge faster than the other. I put in a 12v cutoff switch and all the problems disappear until I drive after dark.

I have a fancy Vanner that works correctly but the 12v stuff bypasses it.

Use a Vanner, it will equal out your 24V and your 12 V needed for your headlights. When you center tape the battery you are taking juice directly from the battery and the all the 24V stuff is pulling from the whole bank and someelse is taking part of its juice so what happens? the 12 V taped one will loose its juice faster and sometimes it is the fastest way to kill that battery. HTH